Such forklift trucks are called “piggy-back” forklifts or “truck mounted” forklifts. Because of the fact that they are transported on other vehicles and trailers, they are, of necessity, of lightweight construction. A typical example of such a forklift is that disclosed in GB Patent Specification No. 2259292 (Moffett Research and Development Limited). A problem arises when they have to load and unload onto platforms where added reach and capacity is required. It is well known to have forklift trucks with means for altering the reach of the forks relative to the mast, either by providing a pantograph linkage or other mechanism between a fork carriage supporting the forks and the mast. European Patent Specification No. 0367356A (Kool Beheer BV) discloses such a pantograph arrangement. Another arrangement comprises using extendable or telescopic forks to provide the reach, such as disclosed in GB Patent Specification No. 1575911 (George E Herbertson). Various other arrangements have been provided, all of which, to a greater or lesser degree, facilitate extendable reach.
However, there are problems associated with this extended reach. Because of the relatively lightweight of the forklift, when the capacity is exceeded by trying to lift something at extended reach, the forklift will overbalance. When extra reach is required, extra forklift capacity is also required. Until now, the extra capacity could only be found by increasing the weight of the forklift. However, the weight of the forklift is critical and so until now, the extended reach across trailers or other platforms with these types of forklift has been limited. In this specification, the term “platform” is used to cover not just simply platforms per se, that is to say, rigid platforms, but indeed trailers, trucks and the like, onto and off which loading is required.
A further problem with the additional loading is that, when the forks are extended, this causes the mast to bear against the platform making it virtually impossible to tilt or side-shift the mast making it very difficult to remove loads from platforms. Further, any form of side shift of the mast becomes almost impossible because the load is such, very often, to bear the forks down against the platform and effectively jam the forks. Various arrangements have been provided to overcome these problems, including the provision of rollers beneath the forks, however, while these solutions have afforded some advantages, they have not allowed the forklift truck to operate in the correct manner. The problem is that the forklift tends to tip under the load. Anything which would prevent this is desirable.
The present invention is directed towards overcoming these problems.